1. Field
This invention relates to beam steering of a phased array antenna and, in particular, to steering control systems in which the total phase shift to the signal applied to the antenna elements is sensed and adjusted by means of feedback circuitry to accurately control the beam direction.
2. Prior Art
Phased array antennas generally comprise a plurality of antenna elements in which the phase of the signal supplied to each element is controlled by one or more phase shifters. Commonly, the phase shifters are controlled by open-loop commands. That is, manual or automatic means sends a commanded steering angle to a unit, usually referred to as a beam steering computer. This unit generates instructions for the phase shifters. These instructions, when executed, cause a transmitted signal to be radiated in a beam with nominally the desired beam pointing direction. No feedback of actual beam pointing direction occurs so that pointing errors caused by errors in producing the required phase shifts are undetected and are not compensated.
High accuracy beam pointing control has been obtained in closed-loop tracking systems. In the case of a radar application, a measure of the reflecting object's angular displacement from the beam boresight direction is fed back to the beam steering computer. This enables corrections to be computed for the phase shifter instructions so that the beam boresight direction is brought into alignment with the direction to the reflecting object. Similarly, in the case of a full-duplex communications application, a measure of the first party's beam displacement from perfect alignment is obtained by the second party and is communicated back to the first party. This allows the first party to correct the phase shifter settings and thus the beam pointing angle. In both these applications external aids to sensing the pointing error are required; that is, the presence of a concentrated target in the radar case and the cooperation of a second party in the communications case. This lack of self-sufficiency is a disadvantage.
Another type of prior art system which is self-sufficient and which employs a type of internal feedback is shown in FIG. 1. In this system, the signal to be applied to the antenna elements is first supplied to an antenna distribution system 112 at port 101 where it is then passed through a series of phase shifters 103A through 103D along the distribution system. The signal is coupled to the antenna elements 104A through 104D by means of couplers 105A through 105D which are spaced along the distribution system between the phase shifters. Each coupler is equipped with a termination to absorb reflections and an attenuator to aid in matching the elements to the distribution system.
In the operation of this system, the signal to be transmitted is applied to the antenna element by way of the distribution system, phase shifters and couplers. The phase shifters are identical and each receives the same control signal. Accordingly, each imparts the same phase shift to the signal in the distribution system. The signal arriving at any one antenna element is shifted in phase with respect to the previous element along the distribution system by the phase shift of a single phase shifter. This is the requirement for steering the beam off boresight by a spatial angle which is directly related to the phase angle imparted by each phase shifter.
Phase shifters 109 and 113 and phase detector 106 comprise the control circuitry which sets and corrects the angle of the phase shifter 103A through 103D and thus controls the beam direction. In the operation of this circuitry, a command signal applied to port 114 sets phase shifter 113 to a particular phase shift setting corresponding to the desired beam steering angle. A signal at the same frequency as the signal to be transmitted is applied to the port 108, divided and passed through the phase shifters 109 and 113 to the input ports of phase detector 106. The output of phase detector 106 is passed through phase shifter control loops 107A through 107D and 110 to control the phase shift of shifters 103A through 103D and 109, setting them all to the same angle.
The phase of the signals arriving at the input ports of the phase detector will have the same phase angle if phase shifter 109 is at the same angle as phase shifter 113. This condition will result in zero output from the phase detector. If all the phase shifters are of the latching type, they will continue to shift in phase until the control signal goes to zero. Since phase shifter 109 is chosen to be identical to phase shifters 103A through 103D, all of these phase shifters will be continually shifted in phase by the output of the phase detector until their phase is equal to that of phase shifter 113, which is set to the desired phase.
The feedback in this circuit is provided by means of a "dummy" phase shifter 109 which is assumed to be at the same phase angle as phase shifters 103A through 103D when the system is at rest because all receive the same control signal; however, as a practical matter, there are differences caused by manufacturing tolerances and different environmental conditions. In addition, the nonuniformities in the antennas produce different reflections which result in different phase shift between the dummy phase shifter 109 and those used to control the beam direction. As a result, there is no feedback to the control circuit or correction of the actual phase shift imparted by the beam steering phase shifters 103A through 103D.